Faraday to Jacob Herbert   30 October 1840

Report of Professor Faraday on the results of his experimental observations in relation to the Edystone Light.

Royal Institution | 30th October 1840.

Sir,

Having read, and considered, the following papers¹ relating to the Edystone Light, and made some experimental observations on the subject, I beg to offer you such judgment as I can form, to be submitted to the Board of the Trinity House, at it’s request.

As the consumption of oil in a four wick’d concentric lamp is probably never more than sixteen times that of an Argand lamp, the oil consumed by the Dioptric arrangement and the 24 reflector arrangement may be taken as 2 to 3. The two kinds of lamps are, perhaps, not very different in the degree of smokiness to which their flames are liable, and, if the results of the combustion were retained in the lanthorn though I think the French lamp would soil the windows and other important parts of the apparatus least, yet I should not be prepared with a very strong opinion on that point. But considering the facility with which the results of the combustion of the single lamp may be conveyed out of the lanthorn, by an arrangement something like that adopted in the Gibraltar apparatus, I think the single central lamp may be made very much to surpass the 24 reflector lamp, in relation to the cleanliness of the inside of the lanthorn, and the freedom of the windows from dimness, dependent on moisture and smoke.

With respect to the light:- There is no doubt, that, when a four-wicked lamp is used, a first class refractor gives more light from it than a second class refractor; for if the two refractors were in their most perfect state, this must happen, from the greater divergence of the beam from the second class or nearest refractor. That eminent philosopher and practical man, M. Fresnel, makes this proportion 1.23 to 1.; by a comparison of two lamps (a three wick’d and a four wick’d lamp) with the same second class refractor. I endeavoured, at the Trinity House, to compare the light from the same four-wick’d lamp after passing through two refractors, namely, one of the 1st Class and one of the 2nd. The lights were, as nearly as I could judge, as 20 to 15, or as 1.33 to 1. This is as near to M. Fresnel’s results, obtained by a different process, as the kind of trial will admit of.

The next point with respect to the light is the comparison of the 24 reflectors with the four-wick’d central lamp in the midst of second class refractors. There are many difficulties in the way for this kind of comparison; to some of which M. Fresnel refers. I have no doubt of the superiority of the reflector light, at the axis of the beam over, even, the best first class refractor furnished with its proper lamp; but, in a fair comparison, we must consider the light, not as it is at 24 favourable places on the horizon but, round the whole circle. Each reflector has to do duty over 15°; and the intensity of light at the centre and at the edges of these 15° is very different. M. Fresnel estimates the difference as being 1 to 10; that is, the light at any place only 7½ degrees from the axis of the reflected beam is but one tenth of what it is in the axis. I have made some experiments with the reflector which was used lately at Purfleet, and which I understand to be a very good one, and the average of the results gives the intensity of the light at the edge of the 15°, as one seventh that at the centre. I also made observations at a distance, intermediate between these two points, and found the average intensity, half of that at the axis of the Beam. According to these results, therefore, we may take the average of the light of this reflector over the whole 15°, as 0.5476, or rather more than one half of that at the centre of the beam.

Now as the light at the centre equalled 224 Argands in my trials, so this average of 0.5476 equals the light of 122 Argands, for this width of 15°. The light is in truth somewhat more than this; because a reflector throws a little light outside of the 15°, and this tends to increase the brilliancy of the beam from the neighbouring reflectors; i.e. where two collateral beams overlap they strengthen each other in illuminating power. Fresnel makes the light at the centre of a reflector beam equal to 300 lamps and at the edge of the 15° = 30 lamps;- if therefore the light were uniform in it’s diminution this would give an average of 165 lamps for the whole of the 15°.

I attempted to measure the intensity of the light thrown by the English second class refractors, at the Trinity House, with the four-wicked lamp in the focus; but in consequence of the irregularity of action of the ribs of the refractors, the beam cast on the wall was so various in intensity, in different parts of it’s height, (of 30 inches) that no safe conclusion could be drawn. The nearest result I could bring out, was equal to 144 Argand lamps. M. Fresnel, using of course a French apparatus, brings out a result of 230 Lamps.

Thus my trials do not set the refractor so much above the reflector as M. Fresnels results do; for with him the ratio is as 230 to 165 or as 100 to 71.7:- but then it is to be remembered that I had not such refractors as he had; and, further, that these experiments have always much uncertainty about them, because of the general character, and the circumstances, of the mode of observation; and because of the difference of colour in the lights. My results make the refractor light of the proposed arrangement surpass the reflector light in the proportion of 144 to 122 or as 100 to 84.7; and that with a consumption of oil of 2 only for the greater light and of 3 for the smaller; the further advantages of equality of light round the whole horizon and of removal of the smoke and moisture of the lamp flame being also obtained.

I therefore, even upon the strength of my own imperfect results, agree with M. Fresnel in both his conclusions².

I have the honor to be | Sir, | Your obliged and obedient Servant | (signed) M. Faraday

Jacob Herbert, Esqre. | Secretary; | &c. &c. &c. | Trinity House.